Buying guide · 5 min
BMX for kids: which size to buy by age and height
Size chart by height (12" to 20"), weight, brakes and safety to get a child’s first BMX right.
Guide updated
Quick answer: On a kids’ BMX, the child’s height rules — not their age: 12" for roughly 85–100 cm, 14" for 95–110 cm, 16" for 105–125 cm, 18" for 120–140 cm and 20" from about 135–140 cm (with a short top tube). Prioritise a light bike the child can lift on their own, a rear brake lever their hand can reach, and a certified helmet from day one. Do not buy “room to grow”: an oversized BMX is not learned on, it is survived.
Buying a child’s first BMX looks easy until you reach the shop: sizes are given in wheel inches, ages do not match between brands and everyone has an opinion. This guide orders the decision the way clubs do: height first, then weight and brakes, and only then the model.
Why BMX sizing does not work like a normal bike
On a city bike you just raise the saddle. In BMX the child rides standing up, jumps, turns and brakes with their body: if the bike is too big, they cannot pull the brake hard, the bars sit too far away and every fall hits twice as hard. If it is too small, their knees hit the bars and it is outgrown in two months.
The measurement that matters is wheel diameter in kids’ sizes, and from 20" upwards the top tube (TT) length takes over, exactly as on adult bikes. Age is only a secondary reference.
Kids’ BMX size chart (guideline)
| Height | Approx. age | Wheel | Notes |
|---|---|---|---|
| 85–100 cm | 2–4 years | 12" | Often pedal-less or very light; balance and gentle pump track riding |
| 95–110 cm | 3–5 years | 14" | First pedal bike for many children |
| 105–125 cm | 5–8 years | 16" | The most common “starter” size for skateparks and tracks |
| 120–140 cm | 7–10 years | 18" | The bridge between kids’ bikes and 20": avoids jumping straight to a heavy 20" |
| 135–150 cm | 9–12 years | 20" (TT 18.5–20") | Already a “real” BMX: choose a short top tube |
| Over 150 cm | 12 and up | 20" (TT 20–20.5") | Use the adult guide: how to choose your first BMX |
The 30-second shop test
Standing over the bike with the pedals level, the child should be able to move it back and forth without hitting their knees on the bars, reach the brake lever with two fingers and touch the ground with at least the tips of their feet when seated. If any of the three fails, it is not their size, however much they will “grow into it”.
What to look for (in this order)
1. Weight: the factor parents miss and kids feel
A cheap steel kids’ BMX can weigh nearly half as much as the child. If they cannot lift it off the ground or push it up a kerb, they will not ride it. Compare the weight on the spec sheet: in 16–18" sizes, aim as close as possible to 8–10 kg. Aluminium and chromoly weigh less than hi-ten steel, but at this level total weight matters more than the “pro” material.
2. Brakes their hand can actually pull
An effective rear brake is non-negotiable (U-brake or V-brake depending on size) with an adjustable or child-sized lever. When learning at the track or skatepark, the rear brake is their safety net. Check the lever does not sit a palm’s width from their closed hand.
3. Freestyle or racing? Not the same bike
- Freestyle (skatepark, pump track, street): wide tyres, steel frame, built for tricks and knocks. The default choice if they “want to do tricks”.
- Racing (dirt track and club): light aluminium frame, skinny knobbly tyres and its own sizes (Micro, Mini, Junior, Expert…). Only if they will train at a club: racing sizes are not chosen from the chart above but from the manufacturer’s chart and the club’s advice. Find the track first in the directory of BMX tracks in Spain.
4. What they do NOT need (save there)
- A flashy rotor/gyro on very cheap bikes: it adds weight and maintenance; unnecessary for starting out.
- Pegs from day one: first riding, braking and pumping; pegs arrive with grinds.
- “Pro spec” at age 7: you pay for parts they will not notice. Better a right-sized bike and a good helmet.
Budget: how much to spend, and on what
- Basic supermarket range: only valid as a first contact in 12–16" if the weight is manageable; check the brakes and tighten everything when you get home.
- Entry-level specialist brands: the sensible tier for 16–18": better weight, brakes and bearings.
- Serious starter bikes: complete 20" bikes and entry-level kids’ race bikes; here it pays to compare with the first BMX guide.
- Second-hand, at any age: a good used BMX beats a bad new one. Follow the part-by-part checklist in buying a used BMX without the nasty surprises and check it is not stolen.
And always keep part of the budget for the helmet. It is not optional, and not “for when they learn to jump”.
Safety: mandatory from day one
- A certified helmet (EN 1078 in Europe), properly fitted: how to choose one and which certification to look for, in the BMX helmets guide. In racing a full-face helmet is mandatory, and at many tracks for training too.
- Gloves and shin pads when starting: the classic beginner falls are hands and shins against the pedal. Full detail in BMX protective gear.
- Where to practise: a pump track or a skatepark at quiet times rather than the street. Learn the skatepark and track etiquette together: turns, lines and not cutting across. Find the nearest spot in the track directory.
- A short, successful first session: 30–45 minutes of riding and pumping beat an afternoon of frustration with YouTube tricks.
The 3 mistakes we see most from parents
- Buying by age (“they are 9, get the 20"”) without checking height or trying the bike.
- Buying big “so it lasts”: it lasts, yes — in the garage. A right-sized bike ridden for two years and resold works out cheaper than an oversized one that makes them quit the sport.
- Spending everything on the bike and nothing on the helmet: the right proportion is the opposite of what almost everyone does.
Clear on the size? Find the nearest track or pump track to break it in, and if you are torn between two models, start with the first BMX guide.
Frequently asked questions
At what age can a child start BMX?
Once they can balance on two wheels they can ride a gentle pump track, often around age 3–4 on a 12–14" bike. For a racing club, many run schools from age 4–5; check with your nearest track.
A 16" or 18" BMX for a 7-year-old?
It depends on their height: under about 120 cm, go 16"; between 120–140 cm, an 18" is better than jumping to a heavy 20". The 18" is the bridge size that avoids the most common mistake.
Do they need a front brake?
Not to start. The rear brake, well adjusted and within reach, is the one they will use as their safety net. Many freestyle BMX bikes do not even fit one.
Can the same BMX be used for the skatepark and for racing?
For trying both, yes, with a freestyle bike. If the child joins a racing club and competes, they will need a race bike in their category size: the geometry and tyres are different.
How long does a kids’ BMX last before it is outgrown?
As a rule of thumb, one size lasts about 1.5 to 2.5 years depending on growth spurts. That is why second-hand — both buying and reselling — fits this sport so well.
